PROJECT SUMMARY The overarching goal of this R01 application (PA-18-278: Innovations in HIV Testing, Adherence, and Retention to Optimize HIV Care Continuum Outcomes) is to advance the science of mobile health approaches to increase uptake of repeat HIV self-testing (HST) and linkage to HIV-related care with populations that underperform on these steps of the HIV care continuum. We propose an intervention entitled ?WeTest- WeLink? that builds on years of formative work with MSM in China (R34MH106349), demonstrating promising effects from a pilot RCT and strong indication of intervention acceptability, feasibility, and cultural sensitivity. The intervention uses the ?WeChat? mobile app platform, which offers multiple features built into the app that facilitate health information delivery and communication channels (e.g., capacity for private texts, group chats, video sharing, GPS, instant messaging, real-time audio and visual communication). Guided by the Information- Motivation-Behavioral (IMB) model and Minority Stress Theory, we will employ a user-centered design process to refine and expand app features to support repeat HST uptake, behavioral risk reduction, stigma coping strategies, and self-efficacy to link to HIV care. We will use an Effectiveness-Implementation Hybrid Type 1 design consisting of a three site, 2-arm RCT to test HST and linkage to HIV related care outcomes as well as qualitative research to examine implementation and scalability. We will recruit 1,800 HIV negative MSM in three cities with HIV high-prevalence among MSM ? Chengdu (9%), Suzhou (13%), Wuhan (11%) ? allocated to the intervention (access to the WeTest-WeLink app) or control group (education about HST and passive referral to HIV care for individuals who test HIV positive). We will assess participants at 6-, 12-, and 18 months to measure intervention effects on primary outcomes: repeated use of HST (including photographic confirmation) and linkage to care for individuals who test HIV-positive. Secondary outcomes include sexual risk behaviors and use of HIV prevention services, and we will conduct mediation analysis to examine theoretical mechanisms of behavior change. We will qualitatively assess intervention-related process characteristics that enable and/or impede implementation and scalability informed by the Consolidated Framework for Implementation Research (CFIR). This project will provide the first known evidence for a mobile health approach to optimize both HIV testing and linkage to care as part of a single intervention continuum with MSM. Such findings can be crucial for optimizing the care cascade in populations that underutilize HIV services, such as MSM in China and elsewhere in the world where HIV testing and linkage to care services are sub-optimal.